1. Field of the Invention
This invention relates to a shift control system of an automatic transmission for a vehicle, and more particularly to improvements in a shift control system of an automatic transmission for a vehicle, wherein the shift control system includes at least a first and a second transmissions capable of automatically switching speeds, and the first and the second transmissions are shifted to thereby achieve multi-speed shifts.
2. Description of the Prior Art
Along with a rapid spread in use of the automatic transmissions for motor vehicles in recent years, there have been commonly adopted such transmissions wherein a so-called over-drive device, in which a transmission gear ratio is less than 1, is connected as the second transmission, in series to the first transmission capable of automatically switching the shift speeds in association with a vehicle speed, a throttle opening, etc.
Futhermore, there is also known such a transmission wherein, based on a function of the second transmission capable of switching between lower speed and higher speed as the above-described over-drive device, shift controls shown A portion in FIG. 4 for example are performed, so that multi-speed shifts of forward six speeds can achieved. This transmission is of such an arrangement that a shift of the second transmission is actively cooperated with a shift of the first transmission, whereby the first transmission and the second transmission are shifted simultaneously or alternately, so that multi-speed shifts can be achieved. The above-described arrangement makes it possible that the existing automatic transmission is utilized as the basis, and changes in design are minimized for the manufacturing advantage, so that multi-speed shifts can be achieved. As the result, such many advantages can be offered that the fuel consumption rate is improved, the power performance is bettered, the burden of frictional materials is relieved due to making the speed shifts into multi-speed shifts, and the like.
However, in the above-described automatic transmission wherein the first transmission and the second transmission are shifted simultaneously or alternately to achieve the multi-speed shifts, there has been such a disadvantage that it is difficult to obtain satisfactory shift characteristics of all of the shifts accompanied by the high gear shift of the second transmission, by a mere change of an acting force of a frictionally engaging device for the high gear shift of the second transmission in accordance with the throttle opening as in the common practice of the up shift, for example.
Namely, in the case of the shift from the first speed to the second speed, the first transmission remains as it is and only the high gear shift of the second transmission realizes the up shift of the automatic transmission as a whole, as shown in FIG. 4 for example. Furthermore, in the case of the shift from the third speed to the second speed for example, the first transmission is low gear shifted and the second transmission is high gear shifted, whereby the down shift of the automatic transmission as a whole is realized. In this case, if the acting force of the frictionally engaging device to the high gear shift of the second transmission is set at an acting force capable of obtaining the satisfactory shift characteristics in the former case for example, then, in the latter case of the shift, the acting force of the engaging device becomes excessive, whereby the shift characteristics are deteriorated. On the contrary, if the acting force of the engaging device is set at an acting force capable of obtaining satisfactory characteristics in the latter case of the shift, then, in the former case of the shift, the start of the high gear shift is delayed or the time point of completion of the high gear shift is so delayed that the high gear shift of the second transmission is completed after the low gear shift of the first transmission, thus presenting such a disadvantage that the driving feeling of the satisfactory down shift cannot be obtained.